Voltage-gated calcium (Ca) channels serve two major roles, an electrogenic role by passing current and thereby affecting membrane potential and a regulatory role because they are selectively permeable to Ca, an important signaling molecule. Changes in global intracellular and local Ca concentrations lead to the activation of soluble a dependent proteins and channels and thereby control diverse cell processes including membrane excitability, cell death, growth, vesicular release, and synaptic plasticity. Regulation of Ca is, therefore, critical and disruption of this regulation is pathophysiological. T-type Ca channels, one family of voltage-gated Ca channels, are widely expressed in the brain. They serve both an electrogenic and regulatory role in that they contribute to lowthreshold Ca spikes, pacemaking, rebound burst firing as well as producing a steady Ca influx near the resting membrane potential and low amplitude Ca oscillations. They have been implicated in several neurological disorders including but not limited to absence seizures, neurogenic pain, sleep disorders, and ataxia. This studys aimed at understanding permeation through these channels in an effort to better understand how they can participate in cellular physiologies and pathophysiologies. Specifically subconductance behavior in these channels will be examined using single channel patch clamp techniques to provide a quantitative measure of Ca flux through these channels. First the number of conductance states and the voltage dependence and kinetics of each for CaV 3.1, a T-channel isoform broadly expressed in the brain, will be determined. Second, conditional probabilities for all conductance levels will be measured to test the hypothesis that subconductance states arise From partial opening of the pore during gating. Lastly because T channels are thought to have complex permeation "rules" that are dependent on other ions, the relative amplitude and kinetics of each conductance state will be measured in both the presence of different permeant cations, Ca, barium, and monovalent cations. In the second aim quantitative differences between CaV 3.1 and CaV 3.2 will be assessed. These two isoforms display complementary expression patterns in the brain and are differentially expressed during development. Identifying similarities and differences between these two isoforms will aid in understanding how each contributes to the cellular physiologies in which they participate. Data obtained here will be helpful in understanding the role of these channels in the variety of neurological diseases in which they play a role and potentially in developing drugs that selectivity target these channels and spare other voltage-gated channels.